Microbiology of Bioremediation

Bioremediation is the use of biological agents such as bacteria and fungi to degrade or detoxify pollutants in a given environment. It may involve the use of specific organisms introduced into the polluted environment or, more commonly, it may take advantage of organisms already present, possibly adding nutrients to encourage their growth.

Pollutants

Pollutants from domestic and industrial wastes have often been dumped into the environment as a matter of convenience. Fortunately, most organic compounds of natural origin can be degraded by one or more species of soil or aquatic organisms under appropriate conditions for microbial growth. As oil spills dramatically demonstrate, however, some natural materials can cause devastating effects before they are degraded. Synthetic compounds are more likely to be degradable if they have a chemical composition similar to that of naturally occurring substances. Xenobiotics, synthetic compounds that are totally different from any that occur in nature, often persist for long periods of time. This is because microorganisms are unlikely to have enzymes necessary for degrading foreign substances; such an enzyme would not give them a competitive advantage in a natural situation.

Relatively slight molecular changes markedly alter the biodegradability of a compound. Perhaps the best-studied example involves the herbicides 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). The only difference between these two compounds is the additional chlorine atom on the latter. When 2,4-D is applied to the soil, it disappears within a period of several weeks, as a result of its degradation by a segment of the microbial population in the soil. When 2,4,5-T is applied, however, it is often still present more than a year later (figure 31.10). Its persistence is apparently due

Figure 31.10 Comparison of the Rates of Disappearance of Two Structurally Related Herbicides, 2,4-D, and 2,4,5-T The addition of a third chlorine atom in the 2,4,5-T molecule blocks the enzyme that degrades the substance, so that compound remains in the environment.

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